Hi, my wife and I live in western New York in a house we bought last year. Our house is well insulated and weather-tight except for one area. This area was once attic space that the previous owner converted to a loft overlooking the kitchen. Itís ceiling is framed with 2x6ís and the finished ceiling (drywall) is sloped/vaulted. There is currently R-19 fiberglass jammed into the 5.5Ē rafter cavity with a cheap Styrofoam type baffle that is most likely crushed in several spots. This area is the only one that continually melts snow in the winter resulting in ice dams. Heat during the summer also builds up in the loft due to the roof heat coming into the house.

I would like to take the dry wall down and replace the fiberglass with rigid board. This is the only way I can get close to R-38 without lowering the ceiling too much (peak of the ceiling is approx 4í above the loft floor). I have looked into spray expanding foam, but even that doesnít give me the R-value without adding more rigid board. The only thing I am concerned with is how much the rafters expand due to summer humidity. If I re-insulate during the summer tightly squeezing the insulation boards between the rafters, will the foam boards become loose during the winter when the rafters dry out and possibly shrink? Also, do I need to get the insulation value up to R-38, or is R-30 enough to keep the snow on the roof?

Sounds good,dont worry about the wood movement, it has most likely done it's dance.
Fill the bays and try to get at least a 1/2 inch foam board to span all the rafters.You can nail these to the rafters enough to hold them up. This creates a thermal bridge and seals the roof.Then sheetrock over that ,just use longer screws.No vapor barrier is needed in this installation.Have fun.....

There is also a method of insulating called the "Ice House" wherein you leave a 1" space next to the roof sheathing and fill the rest of the space with insulation. This would also require soffit vents and a vent-a-ridge so you can get full air flow between each rafter. This method will also keep your shingles cooler in the summer and, if they are asphalt, it will make them last years longer.
Glenn

Just make sure like Glenn said that you do leave a gap between the rigid board and the underneath side of the roof sheathing. It can get ugly on the inside with mold and ugly on the outside with curled shingles.

This technique you could increase the profundity of insulation between the rafters. Alternately, you could insulate the roof deck under the shingles using several inches of foam in addition to foam board or other types of insulation placed between the rafters below.